CN1982946B - Laser display apparatus - Google Patents

Abstract

The invention provides a laser display device capable of suppressing spots. The laser display device includes: a semiconductor laser including a first mirror and a second mirror, which are separated by a cavity distance and form a cavity; and a space adjuster that adjusts light transmitted through the first mirror of the semiconductor laser to An image is displayed on the screen, where, when the coherence length lc and the effective reflectivity R are defined by the following equation: [equation]

Wherein d is the cavity distance of the semiconductor laser, _R is the reflectivity of the first mirror, and _R is the reflectivity of the second mirror, and the coherence length _lc satisfies the following inequality: [Inequality] 0<l _c ≤0.85 [cm].

Description

laser display device

technical field

The present invention relates to a laser display device having a laser light source, in particular, a laser display device capable of suppressing speckles.

Background technique

In general, laser display devices can be easily applied to projection displays because laser beams are parallel. In addition, the intensity of the laser beam is greater than that of light produced by other light sources, and thus the laser can be used to present clear, large-screen images.

Laser display devices use highly coherent laser light, causing speckles. Speckle involves randomly generated coherent noise due to the high coherence of laser light sources.

For example, when the image illustrated in FIG. 1A is presented on a screen by a laser display device, the image appears as illustrated in FIG. 1B due to the spots described above.

US Patent No. 5,274,494 discloses a "Speckle Suppression Illuminator" which suppresses the speckles described above.

The "speckle suppression illuminator" comprises a Raman cell, which is a cavity storing a gas for scattering incident light, in front of a light source radiating coherent light having a first wavelength. Therefore, the illuminator provides a spatially coherent but temporally incoherent laser beam to reduce the intensity and area of occurrence of speckle.

However, since the Raman unit is on a path where the laser light source radiates light, the optical structure of the laser display device is complicated, and the number and cost of manufacturing processes are increased.

Contents of the invention

The present invention provides a laser display device that suppresses mottle without including any additional components.

According to one aspect of the present invention, the provided laser display device includes: a semiconductor laser, including a first mirror and a second mirror, which are separated by a cavity distance and form a cavity; The mirror transmits the light to display an image on the screen, where, when the semiconductor laser coherence length _lc and effective reflectivity R are defined by the following equation:

[equation]

$\mathrm{<span\; class="notranslate">\; lc</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; d</span>}\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="R"\; filenames="H061C1562720060904E000031.png"\; state="\{\{state\}\}">R</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}}$

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="R"\; filenames="H061C1562720060904E000061.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

where d is the cavity distance of the semiconductor laser, _R1 is the reflectivity of the first mirror, and _R2 is the reflectivity of the second mirror, and the coherence length _lc satisfies the following inequality:

[inequality]

0<l _c ≤0.85[cm]

The effective reflectivity R of the semiconductor laser may range from 0.1 to 0.3.

Description of drawings

The above and other features and advantages of the present invention will become apparent by describing in detail exemplary embodiments thereof, with reference to the accompanying drawings, in which:

Figure 1A is the original image;

Figure 1B is the image of Figure 1A with blobs;

FIG. 2 is a schematic diagram of a laser display device according to an embodiment of the present invention.

FIG. 3 is a graph illustrating coherence length versus effective reflectivity of a semiconductor laser.

Figure 4 illustrates how the spot contrast (Cs) is measured in a semiconductor laser;

5A to 5D are photographs of images when the effective reflectance (R) is 90.5%, 66.6%, 18.7% and 5.4%, respectively;

Figure 6 is a graph of spot contrast (Cs) versus effective reflectance; and

Figure 7 is a plot of speckle contrast (Cs) versus coherence length.

Detailed ways

The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.

FIG. 2 illustrates an optical arrangement of a laser display device according to an embodiment of the present invention. Referring to FIG. 2 , the laser display device scans laser light on a screen 30 and includes a semiconductor laser 10 and a space regulator 20 that adjusts incident light to display an image on the screen 30 .

Semiconductor laser 10 comprises first and second clad layer (clad layer) 11 and 15, is arranged on the active layer 13 between first and second clad layer 11 and 15, and between active layer 13 and first and second clad layer First and second mirrors 12 and 14 on the sides of 11 and 15 for forming cavities. The semiconductor laser 10 radiates visible light having a wavelength ranging from about 400 nm to about 700 nm. The first mirror 12 and the second mirror 14 are separated from each other by a cavity distance d.

Therefore, when power is applied to the semiconductor laser, the light beam generated by the active layer 13 resonates between the first mirror 12 and the second mirror 14, and only light of a predetermined wavelength is amplified by the first mirror 12 and the second mirror 14 and transmission.

The space adjuster 20 adjusts the light transmitted through the first mirror 12 to display an image on the screen 30 .

In this embodiment, by controlling the cavity distance d and the effective reflectivity R of the semiconductor laser 10 to change the coherence length lc, the speckle contrast on the screen 30 is reduced. The relationship between the semiconductor laser 10 and the coherence length lc, which determines the spot intensity on the screen 30, will now be described, followed by the optimization of the reflectivity R of the semiconductor laser 10 and the cavity distance d.

The coherence length lc having a narrow width equal to the width of the semiconductor 10 can be expressed by Equation 1.

[Formula 1]

$\mathrm{<span\; class="notranslate">\; lc</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; d</span>}\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}}$

where c is the speed of light, F is the finesse of the Fabry-Perot cavity, _VF is the modulus at a given frequency, d is the cavity distance, and R is given by The effective reflectivity of the semiconductor laser 10 defined by Equation 2.

[Formula 2]

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="R"\; filenames="H061C1562720060904E000061.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

Where: R ₁ is the reflectivity of the first mirror 12 and R ₂ is the reflectivity of the second mirror 14 .

Referring to Equation 1, the coherence length lc representing the distance at which coherence is maintained depends on the cavity distance d and the effective reflectance R of the semiconductor laser 10 . Thus, as the reflectivity R and cavity distance d decrease, the coherence of the radiation beam decreases and thus the spot intensity on the screen 30 decreases.

FIG. 3 is a graph illustrating coherence length versus effective reflectivity of a semiconductor laser. Specifically, FIG. 3 illustrates the coherence length lc and the coherence factor Fc of GaN-type semiconductor lasers when the effective reflectances are 90.5%, 66.6%, 18.7% and 5.4%, respectively.

Referring to FIG. 3, as the effective reflectivity of the semiconductor laser decreases, lc and Fc decrease.

Fig. 4 is a schematic diagram of a speckle contrast measuring device for a semiconductor laser. The speckle contrast measurement device includes a GaN type semiconductor laser 41 mounted on a cooling device 43, the effective reflectivity of which is 90.5%, 66.6%, 18.7% and 5.4%, respectively, and the cavity distance is 650 μm. The semiconductor laser 41 is driven by a laser driving source 45, and the far field of the semiconductor laser 41 is projected on a screen 47 without a lens system. The image on the screen 47 is taken with a camera 49, such as a digital camera with a manual function, using F10 aperture and -2.0EV exposure, and the spot contrast Cs is calculated using formulas 3-5.

[Formula 3]

$\mathrm{<span\; class="notranslate">\; Cs</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}{\mathrm{<span\; class="notranslate">\; avg</span>}}$

in

[Formula 4]

$\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; avg</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; N</span>}}}$

and

[Formula 5]

$\mathrm{<span\; class="notranslate">\; avg</span>}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; N</span>}}$

where I(n) is the light intensity, N is the number of data, avg is the average light intensity, and б is the standard deviation.

5A to 5D are photographs of images taken when the effective reflectance is 90.5%, 66.6%, 18.7% and 5.4%, respectively. Figure 6 is a graph of spot contrast Cs versus effective reflectance.

FIG. 5A is a photograph of an image projected onto a screen by a semiconductor laser, where the reflectivity R ₁ of the first mirror 12 and the reflectivity R ₂ of the second mirror 14 are both 90.5%. The spot contrast Cs is 0.422, and as illustrated, the image has rough spots.

5B is a photograph of an image projected on a screen by a semiconductor laser, where the reflectivity R ₁ of the first mirror 12 is 66.6%, and the reflectivity R ₂ of the second mirror 14 is 90.5%. The spot contrast Cs is 0.312, and the spots are relatively smaller than those in Fig. 5A.

5C is a photograph of an image projected on a screen by a semiconductor laser, where the reflectivity R ₁ of the first mirror 12 is 18.7%, and the reflectivity R ₂ of the second mirror 14 is 90.5%. The spot contrast Cs is 0.171, and the spots are much smaller than those shown in Figures 5A and 5B.

5D is a photograph of an image projected on a screen by a semiconductor laser, where the reflectivity R ₁ of the first mirror 12 is 5.4%, and the reflectivity R ₂ of the second mirror 14 is 90.5%. The speckle contrast Cs is 0.133, and the image has almost no speckles.

As described above, as the reflectance R of the semiconductor laser decreases from 90.5% to 5.4%, the coherence decreases, and as illustrated in FIG. 6, the spot contrast Cs can be reduced to one-third.

Meanwhile, as shown in Equation 1, the coherence length lc not only depends on the effective reflectivity R, but also depends on the cavity distance d. At the same time, the coherence length lc defined by the cavity distance d and the effective reflectivity R is related to the change of the spot contrast Cs.

Figure 7 is a plot of speckle contrast versus coherence length lc. Referring to FIG. 7, as the coherence length lc increases, the spot contrast Cs increases, becoming level at high coherence lengths. For example, when lc is 0.85 cm, Cs is 0.25 or less, and when lc is larger than 0.85, Cs is larger than 0.25.

Thus, semiconductor lasers can be improved without additional optical components or radio frequency (RF) drivers.

Referring to FIG. 2, that is, the semiconductor laser 10 has a coherence length lc satisfying the following inequality.

[Formula 6]

0<l _c ≤0.85[cm]

The above inequality is determined from the relationship between the coherence length lc and the spot contrast Cs, and when the coherence length lc exceeds 0.85, the spot contrast Cs exceeds 0.25, and thus the spots on the screen become larger.

Meanwhile, the cavity distance d may satisfy the following inequalities while satisfying Formula 6 at the same time.

[Formula 7]

400≤d≤1000[μm]

This inequality is based on the fact that the coherence length lc is proportional to the cavity distance d. When the cavity distance d is kept within the range of Equation 7, the coherence length lc can be reduced. When the cavity distance d is in the range of Equation 7, the speckle contrast can be kept below 20%, even if the effective reflectance R is greater than 0.3.

Meanwhile, under the condition of Formula 6, the effective reflectance R of the semiconductor laser 10 satisfies the following inequality.

[Formula 8]

0<R≤0.62

When the effective reflectance R satisfies the above inequality, the spot contrast Cs is 0.25 or less when the cavity distance d is 650 μm. Therefore, the speckle contrast Cs is greatly reduced compared with the spot contrast Cs of the semiconductor laser which is 0.422 when the effective reflectance is 90.5%, and thus the speckle is suppressed.

In one embodiment of the present invention, the effective reflectance R of the semiconductor laser 10 satisfies the following inequality.

[Formula 9]

0.1≤R≤0.47

When the effective reflectance R satisfies the above inequality, the basic resonance structure of the semiconductor laser 10 is formed, and the spot contrast Cs is 0.2 or less.

As described above, laser display devices can suppress speckle on the screen by controlling the effective reflectivity and/or cavity distance in the semiconductor laser and thus optimizing the coherence length lc, without including any additional components.

Although the present invention has been shown and described in detail with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the present invention. The spirit and scope of the invention are as defined by the appended claims.

Claims (5)

1. A laser display device, comprising: a semiconductor laser comprising first and second mirrors separated by a cavity distance and forming the cavity; and a spatial modulator, modulating the light transmitted through the first mirror of the semiconductor laser to display an image on a screen, Among them, when the coherence length lc and effective reflectivity R of the semiconductor laser are defined by the following equation: [equation] $\mathrm{<span\; class="notranslate">\; lc</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; d</span>}\frac{{\mathrm{<span\; class="notranslate">\; \&pi;R</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; ,</span>$ $\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; ,</span>$ Wherein d is the cavity distance of the semiconductor laser, R ₁ is the reflectivity of the first mirror, and R ₂ is the reflectivity of the second mirror, and the coherence length _lc satisfies the following inequality: [inequality] 0cm < l _c ≤ 0.85cm. 2. The laser display device as claimed in claim 1, wherein the cavity distance d satisfies 400μm≤d≤1000μm. 3. The laser display device as claimed in claim 1, wherein the effective reflectivity R satisfies 0≤R≤0.62. 4. The laser display device as claimed in claim 3, wherein the effective reflectivity R satisfies 0.1≤R≤0.47. 5. The laser display device of claim 1, wherein the semiconductor laser radiates visible light having a wavelength of about 400 nm to about 700 nm.

Images